In an exhaust gas recirculation system (hereafter, EGR system), an exhaust gas discharged from an internal combustion engine is partly returned to an intake side of the engine. An exhaust gas heat exchanger is disposed to perform heat exchange between a coolant and the part of the exhaust gas (hereafter, EGR gas) to be returned to the intake side of the engine, thereby to cool the EGR gas.
In the EGR system, the volume of nitrogen oxide is reduced. Since the EGR gas is returned to the intake side of the engine after being cooled by the heat exchanger, the effect of reducing the nitrogen oxide further improves. If the EGR gas is merely recirculated, the amount of particulate maters emissions and the amount of hydrocarbon emissions will increase according to operation conditions of the engine. That is, the EGR gas has an optimum temperature that can reduce the amount of nitrogen oxide emissions and particulate matters.
Japanese Patent Publication No. 2004-257366 discloses an EGR heat exchanger for an EGR system. The disclosed heat exchanger having EGR cooling passages for cooling the EGR gas by an engine coolant and bypass passages in which the EGR gas is not cooled. The bypass passages are surrounded by air-filled layers so that the EGR gas passing through the bypass passages is not cooled. The EGR cooling passages and the bypass passages are disposed parallel to each other. In the disclosed EGR system, the volumes of the EGR gas flowing into the EGR cooling passages and the bypass passages are controlled by a switching valve that is connected to the EGR heat exchanger in series, thereby to control the EGR gas temperature to the optimum temperature.
In the disclosed EGR heat exchanger, cooling tubes that define the EGR cooling passages and bypass tubes that define the bypass passages are stacked in an inside of a tubular casing. Bonnets are coupled to ends of the tubular casing for fixing the EGR heat exchanger to an EGR gas passage of the EGR system. In the casing, a separation wall is provided between the cooling tubes and the bypass tubes such that the inside of the casing is separated into two spaces.
The cooling tubes are disposed in a first space and the bypass tubes are disposed in a second space. The engine coolant is introduced into the first space, so that heat exchange is performed between the engine coolant and the EGR gas passing through the cooling tubes through the cooling tubes. On the other hand, air is enclosed in the second space, in place of the engine coolant. Namely, air-filled layers are formed outside of the bypass tubes in the second space. Therefore, the EGR gas passing through the bypass tubes is hardly cooled. In this construction, however, it is necessary to air-tightly and entirely fix the separation wall to inner surfaces of the casing.